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Editorial Team
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Asked: 2026-06-10T03:23:07+00:00

Compiling this code using Emscripten: #include <stdio.h> int main() { unsigned long d1 =

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Compiling this code using Emscripten:

#include <stdio.h>
int main() {
unsigned long d1 = 0x847c9b5d;
unsigned long q =  0x549530e1;
printf("%lu\n", d1*q);
return 0;
}

yields (using -g):

  $d1=-2072208547; //@line 3 "minusmul.c"
  $q=1419063521; //@line 4 "minusmul.c"
  var $2=$d1; //@line 5 "minusmul.c"
  var $3=$q; //@line 5 "minusmul.c"
  var $4=((($2)*($3))|0); //@line 5 "minusmul.c"

Executing this using js (SpiderMonkey I believe?) or node, I get the result 3217488896. Executing the native executable (compiled using GCC), I get 3217489085. How would one emulate the x86 unsigned 32-bit integer multiplication using JavaScript?

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1 Answer

  1. Editorial Team

    Emscripten either does not support exact 32-bit multiplication or it is a bug. Since they mention on the homepage that they have software emulation for 64-bit math, I think it’s a bug. I discovered that you can use CHECK_OVERFLOWS and it will discover the overflow. It doesn’t seem to “repair” it though. To make the program finish with CHECK_OVERFLOWS you need to up the count, marked by “XXX” in the generated source.

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